CHAPTER IX. HYBRIDISM.

3. LAWS GOVERNING THE STERILITY OF FIRST CROSSES AND OF HYBRIDS.

We will now consider a little more in detail the laws governing the
sterility of first crosses and of hybrids. Our chief object will be to see
whether or not these laws indicate that species have been specially endowed
with this quality, in order to prevent their crossing and blending together
in utter confusion. The following conclusions are drawn up chiefly from
Gartner's admirable work on the hybridisation of plants. I have taken much
pains to ascertain how far they apply to animals, and, considering how
scanty our knowledge is in regard to hybrid animals, I have been surprised
to find how generally the same rules apply to both kingdoms.

It has been already remarked, that the degree of fertility, both of first
crosses and of hybrids, graduates from zero to perfect fertility. It is
surprising in how many curious ways this gradation can be shown; but only
the barest outline of the facts can here be given. When pollen from a
plant of one family is placed on the stigma of a plant of a distinct
family, it exerts no more influence than so much inorganic dust. From this
absolute zero of fertility, the pollen of different species applied to the
stigma of some one species of the same genus, yields a perfect gradation in
the number of seeds produced, up to nearly complete or even quite complete
fertility; and, as we have seen, in certain abnormal cases, even to an
excess of fertility, beyond that which the plant's own pollen produces. So
in hybrids themselves, there are some which never have produced, and
probably never would produce, even with the pollen of the pure parents, a
single fertile seed: but in some of these cases a first trace of fertility
may be detected, by the pollen of one of the pure parent-species causing
the flower of the hybrid to wither earlier than it otherwise would have
done; and the early withering of the flower is well known to be a sign of
incipient fertilisation. From this extreme degree of sterility we have
self-fertilised hybrids producing a greater and greater number of seeds up
to perfect fertility.